Need of an efficient procedure for the evaluation of ... · PDF fileNeed of an efficient procedure for the evaluation of pozzolanic performance of supplementary cementitious materials

The Indian Concrete Journal January 201680

POINT OF VIEW POINT OF VIEW

Need of an efficient procedure for the evaluation of pozzolanic performance of supplementary cementitious

materials

A. Bahurudeen, Hemalatha M.S. and Manu Santhanam

Several alternative materials, including various wastes and by-products, are available in enormous quantities for use in concrete. However their effective utilisation has been hampered because of inadequate understanding of the materials and inappropriate evaluation practice. This paper describes the current specifications in Indian standards for the assessment of pozzolanic performance of new supplementary cementitious materials. Moreover, different effective methods specified in the international standards, guidelines and earlier research studies on the characterisation of pozzolanic activity are described as well as compared with current practice in Indian standards. This paper also narrates the ways to reach an effective evaluation procedure to assess the pozzolanic performance of supplementary cementitious materials for use in concrete.

INTRODUCTIONConsumption of ordinary Portland cement (OPC) is increasing due to the rapid development in the construction industry. Solid wastes and industrial by-products are used as alternative supplementary cementitious materials (SCM) to achieve durable and sustainable concrete. Performance of any supplementary cementitious material in concrete is significantly influenced by its pozzolanic activity. Most of the alternative cementitious materials are by-products such as slag, silica fume, and bagasse ash from industries, and some of these cannot be directly used as pozzolanic material. Proper characterisation of pozzolanic activity as well as detailed understanding of alternative materials is imperative to reach maximum utilisation of these materials. Direct assessment of pozzolanic activity of waste materials

without characterisation and improper understanding leads to lower pozzolanic reactivity than minimum required as per standard (ASTM 618-12a) [1]. Moreover, waste materials and by-products with inconsistent material properties are obtained from different types of industries. Assessment of pozzolanic activity of different kinds of materials by using the same method is not appropriate and hence test methods based on material characteristics, proper understanding of materials and minimum level of processing need to be well thought-out to attain maximum possible pozzolanic activity. This paper describes the current specifications in Indian standards for the assessment of pozzolanic materials and compares them with international guidelines and standards. Additionally, consideration of characterisation techniques and test methods and their importance in the evaluation of alternative cementitious materials are explained in the paper.

METHODS FOR EVALUATION OF POZZOLANIC ACTIVITY OF SCM

Different standards and guidelines are available for the evaluation of new alternative supplementary cementitious materials. In addition, various effective methods have been described in the previous research studies to evaluate pozzolanic activity of different supplementary cementitious materials. Salient features of different pozzolanic test methods in the Indian and international standards are reported in the following sections.

The Indian Concrete Journal January 2016 81

POINT OF VIEW

Current specifications in Indian Standards

Different test methods including wet chemical analysis, specific gravity, fineness, setting time for material characteristics and lime reactivity test to evaluate the pozzolanic performance of SCMs are described in IS 1727:2004 [2]. Lime reactivity is an important test specified by Indian standards both for the in-situ blended pozzolana (IS 1727) as well as the pozzolana (fly ash) used to manufacture Portland pozzolana cement (IS 3812 and IS 1489 Part 1) [3,4] in the cement factories. Also, this test is used to classify the types of quick setting lime pozzolana mixture (IS 10772) [5].

In the lime reactivity test, 50 mm mortar cubes are prepared with hydrated lime: pozzolanic material: standard sand in the ratio 1:2M:9 by weight as prescribed in the standard (where M is the ratio of specific gravity of the pozzolan to that of lime). In this test, unlike the ASTM standard method (strength activity test), the mix design for pozzolanic activity tests are on an absolute volume basis and therefore the binder-paste volume is maintained constant in both control and test mortars. In addition, a similar procedure is also recommended to determine compressive strength of pozzolana - cement mortar at different ages in this standard. The amount of water is determined as that required to achieve a flow of 70±5% with the flow table dropped for 10 times. After casting, the specimens are demoulded (after 48 hours) and further cured at 50oC for eight days. Lime reactivity is represented as the compressive strength of lime and pozzolana mixture. The primary disadvantage of this guideline is the method of reactivity assessment. This is because unlike cement, strength gain in the lime reactivity test is very slow even after 8 days of curing at 50±2oC. Specimens prepared with highly reactive materials such as

silica fume and rice husk ash are also reported to yield low strength results, and the tests seems to be more favourable for SCMs that have a larger lime content (i.e. the ones that show cementitious characteristics). This can be explicitly seen from Figure 1 which shows the lime reactivity values for some mineral admixtures such as Type ‘F’ fly ash (F1FA), Type ‘C’ fly ash (CFA), slag (AS) and silica fume (SF) from an ongoing research study at IIT Madras. This leads to inappropriate assessment of reactivity of new pozzolanic materials.

Another main drawback in this test is the variation in water content based on flow value. The water requirement for the mixtures in Figure 1 is presented in Table 1. Because pozzolanic performance is directly represented as the compressive strength of cubes in the standard, a reduction or increase in water content considerably influences the measured strength. When this method is adopted for the comparison of pozzolanic performance of different materials, different water contents need to be added during casting as per flow specification. Assessment and comparison of reactivity in terms of strength are therefore biased due to the variation in water content. Moreover, unlike in ASTM C1240-12 for cement-pozzolan mixture, there is no recommendation for usage of a water-reducing admixture in Indian standards with lime-pozzolan mixture [6]. This leads to very high water demand for ultra-fine materials like silica fume to achieve the specified flow. Also, the effect of fineness and water absorbing capacity are not considered in the test procedure. Further, the absence of alkalis and sulphates, that are usually available in a cementitious mixture, leads to a lower potential reactivity from the pozzolanic materials.

It is needless to mention the importance of long term effects in case of pozzolanic materials and that is why all the normally cured pozzolanic activity tests demand only about 75 to 85% of the strength index values at 28 days. But the first revision of IS 1344 (Specification for calcined clay pozzolana) contained a requirement of compressive strength

Table 1. Quantity of materials used for lime-pozzolan mortar cubes (strength results in figure 1)

Type of mix Lime(g)

Pozzolan(g)

Sand(g)

Water(g)

F1FA mixture 150 284 1350 137

CFA mixture 150 338 1350 182

AS mixture 150 373 1350 145SF mixture 150 291 1350 260



at 90 days which is at present deleted in the second revision [7]. This could be reconsidered in future revisions. For the same reason, there is a need for incorporation of 56 and 90 days strength test in the case of the specification for Portland pozzolana cement (IS 1489: Part 1) and also for Portland slag cement (IS 455) [4,8].

IS 1727 suggests Blaine’s air permeability method to determine specific surface area of new supplementary cementitious materials. Nevertheless, this method is not suitable for finding the specific surface area of ultra-fine pozzolanic materials like silica fume and porous materials such as bagasse ash and rice husk ash. IS 15388 suggests the gas adsorption technique of determining the specific surface area of such materials using Brunauer- Emmett-Teller (BET) method [9]. To know the exact degree of calcination for a particular clay sample with specified fineness, differential thermal analysis technique is suggested in IS 1344 which covers the specification for calcined clay pozzolana. From the completion of the endothermic peak, the calcination temperature at which the clay becomes reactive, can be determined. This can be extended for finding the calcination temperature of any new SCM.

Another important issue is the determination of the reactive silica content in the mineral additives. The method of point counting using optical microscope is recommended for finding the reactive silica content of granulated slag used for the manufacture of Portland slag cement (IS 12089). IS 3812 : Part 1 suggests a wet chemical analysis for finding the reactive silica present in fly ash [3,10].

Table 2 summarises the details of tests on pozzolanic materials as per Bureau of Indian Standards.

Although a number of test methods are suggested for pozzolana and pozzolana - cement mortar, detailed performance evaluation of new SCM in concrete including admixture compatibility, heat of hydration, and durability are not addressed in the Indian standards.

DIFFERENT METHODS FOR THE EVALUATION OF POZZOLANIC ACTIVITY IN INTERNATIONAL STANDARDS

Strength activity index test (ASTM standard method)

Strength activity test is suggested in ASTM standards (ASTM C311, 618, 989, 1240) to evaluate the pozzolanic performance of supplementary cementitious materials [1,11,12,6]. The Indian standard suggests the same procedure in the lime reactivity test for different types of materials, whereas

ASTM standards suggest different test procedures based on the reactivity of the materials and their characteristics. This is a prominent feature of ASTM standards to attain exact assessment of the reactivity of supplementary cementitious materials. ASTM C618-12a and ASTM C311-11b describe the requirements and test procedure to find the strength activity index of fly ash. ASTM C989-13 explains the specification for slag activity test with cement and ASTM C1240-12 is the specification for accelerated pozzolanic strength activity index for silica fume. The test procedures described in the ASTM standards are summarised in the following passages.

Six numbers of 50 mm control mortar cubes without replacement are prepared using cement and sand in the ratio 1:2.75 with a water-cement ratio of 0.484 as specified in ASTM C311-11b. The preparation of the moulds, moulding of test specimens and tamping are described in ASTM C109/C109M-12. 20% mass replacement of cement with fly ash or same kind of pozzolanic material is used. A constant water content (242 ml) for control mix is specified in ASTM C311-11b, but the water content for fly ash replaced mortar needs to be determined by trial and error method to get the same flow value as the control mortar with ±5% tolerance. The specimens are cured in saturated lime water at 25oC (to avoid leaching of calcium hydroxide) and compressive strength of specimens is determined after 7 days and 28 days of curing. Strength activity index is calculated as the % ratio of strength of SCM replaced mix to the control mix. For fly ash and natural pozzolanic materials, 75% pozzolanic activity index is recommended as minimum requirement to define as supplementary cementitious material as per ASTM C618-12a.

ASTM C989-13 suggests the same procedure as described above for fly ash to determine slag activity index. However, replacement level of slag is specified as 50% and water content for slag replaced mix is described as the amount of water to achieve a flow of 110±5%. ASTM C1240 suggests an accelerated pozzolanic strength activity index test for silica fume. The replacement level of silica fume is specified as 10%. This standard suggests the same water-binder ratio (0.484) for both control and silica fume replaced mixtures. Specimens are cast and initially cured for 24 hours in the moist room. Afterwards, the specimens are stored at 65±2oC for six days as specified in ASTM C1240. Use of high range water reducing admixture is recommended to attain a flow value of 100 to 115% in both mixes. This is an important feature in ASTM standard, because a replacement level 10% and the use of superplasticiser are suggested in the evaluation of pozzolanic performance of silica fume based on actual concrete applications where generally similar levels of replacement and use of superplasticisers are adopted.


POINT OF VIEW

Table 2. Details on testing of pozzolanic materials as per Indian StandardsMaterial Standards Salient Features

Portland Slag Cement

IS 455 (Specifications)

IS 3535 (Sampling)

IS 4031 – Part 6 (Compressive Strength test procedure)

• Cement : Indian Standard Sand = 1 : 3 by weight• Water based on standard consistency (P) i.e. Vicat needle reading at 5 to 7• Quantity of Water = [(P/4) + 3]% of combined mass of cement and sand• Size of cubes = 70.7 mm• Total number of cubes = 9 (each 3 numbers for 3, 7 and 28 days)• Curing :- Submerged in clean fresh water until testing• Test results :- Not less than 16 MPa at 3 days

Not less than 22 MPa at 7 days Not less than 33 MPa at 28 days

Calcined Clay Pozzolana

IS 1344 (Specifications) IS 1727 (Test procedure)

Lime Reactivity • NCB Lime : Pozzolana : Indian Standard Sand = 1 : 2M : 9 by weight

where M = (Specific Gravity of Pozzolana / Specific Gravity of Lime)• Water required to give a flow of 70 ± 5% with 10 drops in 6 seconds• Size of cubes = 50 mm• Minimum number of cubes = 3 • Curing :- Kept under wet gunny bags till 48 hours; Then cured at 50 ºC and 90 to 100% RH

for 8 days • Tested on 10th day• Test results :- Not less than 4 MPa for Grade I, Not less than 3 MPa for Grade II

Pozzolana – Cement Mortar Strength • Pozzolana : Cement (33 Grade) : Indian Standard Sand = 0.2N : 0.8 : 3 by weight

where N = (Specific Gravity of Pozzolana / Specific Gravity of Cement)• Water required to give a flow of 105 ± 5% with 25 drops in 15 seconds• Size of cubes = 50 mm• Total number of cubes = 9 (each 3 numbers for 7 , 28 and 90 days) • Curing :- Kept at 27 ºC and at least 90% RH till 24 hours; Then demoulded and submerged

in clean fresh water at 27 ºC until testing • For comparison, plain cement – sand mortar cubes are prepared and tested in similar way• Compressive Strength at 28 days for Calcined Clay used in the manufacture of PPC :- Not

less than 80% Strength Index value

Portland Pozzolana Cement (Fly ash based)

IS 1489 – Part 1 (Specifications) IS 3535 (Sampling) IS 4031 – Part 6 (Compressive Strength of mortar)

Refer salient features of Portland Slag Cement

Pulverized Fuel Ash

IS 3812 – Part 1 (Specifications) IS 6491 (Sampling) IS 1727 (Test Procedure)

For Test procedures, refer salient features of Calcined Clay Pozzolana• Lime Reactivity Test results :- Not less than 4.5 MPa• Compressive Strength at 28 days :- Not less than 80% Strength Index value

Quick Setting Lime –Pozzolana mixture

IS 10772 (Specifications)IS 4031 – Part 7(for Compressive Strength of Masonry Cement)

• Masonry Cement = 420 g; Indian Standard Sand = 1440 g • Water required to give a flow of 110 ± 5% with 25 drops in 15 seconds• Size of cubes = 50 mm• Total number of cubes = 9 (each 3 numbers for 7, 28 and 90 days) • Curing :- Kept at 27 ºC and ≥ 90% RH for 48 - 52 hours;

Then demoulded and kept in moist cabinet for 5 days;Then test conducted on 3 numbers of cubes at 7 days;The remaining 6 cubes are kept immersed in clean water until testing on 28th and 90th day

• Test results :- Minimum Compressive Strength required

Tested on Type 1 Type 2 Type 3 7 days 2.5 MPa 1.0 MPa 0.4 MPa 28 days 6.0 MPa 2.5 MPa 1.0 MPa 90 days 8.0 MPa 4.0 MPa 1.5 MPa

Silica fume

IS 15388 (Specifications)IS 6491 (Sampling)IS 1727 (Test on Cement – Silica fume mortar mixture)

For test procedure, refer salient features of Calcined Clay Pozzolana• Compressive Strength at 7 days (Assuming N = 1) :- Not less than 85% Strength Index

value

Granulated Slag IS 12089(Specifications and Testing)

The glass content (reactive silica) determined by Optical Microscopy (refer Appendix A of IS 12089) :- Not less than 85%



In comparison, the use of a constant test procedure (lime reactivity test) in the Indian standard for different types of supplementary cementitious materials may lead to improper assessment of pozzolanic performance, especially with respect to materials such as silica fume that cause a major increase in water demand due to their high fineness.

Table 3 summarises the quantities of materials used in another study from IIT Madras for assessing strength activity index (SAI) values for different mineral admixtures - Type ‘F’ fly ash (F1FA), Type ‘C’ fly ash (CFA), and slag (AS) as per the relevant ASTM guidelines, and Figure 2 shows the SAI results of this study. The results indicate that the mineral admixtures with cementitious characteristics such as slag and Type ‘C’ fly ash perform better than Type ‘F’ fly ash and this resembles the observed results of the lime reactivity test (presented earlier in Figure 2). However, unlike the lime reactivity results, in the strength activity index test, the Type ‘F’ fly ash also performs satisfactorily.

Apart from the strength activity methods, a number of chemical and electrical methods have been proposed and standardised for evaluation of pozzolanic materials. The results from such tests should be used in combination with the strength activity tests to fully understand the pozzolanic potential. These tests are discussed in the subsequent sections.

Frattini test (British standard method BS EN 196-5: 2005)

Frattini test is a chemical method based on British standard EN 196(5)-2005 [13]. In this method, 2 g of pozzolanic material is blended with 18 g of ordinary Portland cement. The blended sample is poured into 100 ml of boiled water in an airtight container and kept in an oven at 40oC for 8 days. After 8 days, samples need to be taken from the oven and immediately vacuum filtered through filter paper using sealed Buchner funnel. 50 ml filtrate is pipetted and used for titration as described in the standard to determine [OH]– and

[Ca]2+ concentrations. The concentrations of hydroxyl ions and Calcium ions from the test should be plotted on the standard solubility curve (See Figure 3) of calcium hydroxide (illustrated in BS EN 196(5)-2005). Calcium hydroxide is consumed in the pozzolanic reaction and leads to the reduction of [OH]– and [Ca]2+ concentration (as equivalent CaO).

For reactive pozzolanic materials, the results are expected to fall below the saturation curve. If results are on the saturation curve or above the curve, the tested sample is deemed to represent no (or insufficient) pozzolanic activity. The observed reduction in [OH]– and [Ca]2+ concentrations is only due to the pozzolanic reaction. This method needs a very controlled procedure and skilled supervision. Controlled temperature and demineralised water are required during

Table 3. Quantity of materials utilized for the preparation of mortar cubes in the ASTM evaluation tests

Type of Mix Cement(g)

Pozzolan(g)

Sand(g)

Water(g)

Control mixture 500 - 1375 242

F1FA mixture 400 100 1375 235

CFA mixture 400 100 1375 250

AS mixture 250 250 1375 230


POINT OF VIEW

titration. The duration of vacuum filtration should be less than 30 seconds to avoid carbon-dioxide absorption from the atmosphere. If not, it influences on the measurement of equivalent CaO. To determine [Ca]2+ concentration, pH of the titrated sample needs to be adjusted to 12.5±0.2 by adding sodium hydroxide solution before titration.

Lime saturation method (Modified Frattini test method)

In the Frattini test, the reactive silica from supplementary cementitious material reacts with calcium hydroxide which is formed from the hydration of ordinary Portland cement. In the lime saturation method, a small quantity (1 g) of pozzolanic material is added to a rich saturated lime solution to facilitate a direct reaction between pozzolanic materials and calcium hydroxide. The primary advantage of this method is that the ratio of calcium hydroxide to pozzolanic material is higher compared to Frattini test. Because of the larger calcium hydroxide content, the complete reactivity of the pozzolanic material can be attained. The lime solution is prepared by dissolving 2 g of lime in one litre of distilled water. Thus, the initial concentration of CaO is known. 1 g of pozzolanic material is added into the saturated lime solution and the mixture is placed in an oven at 40oC. After 3 and 7 days, the sample is filtered, and the filtrate is titrated to determine [Ca]2+ concentration (as equivalent CaO) using the same procedure as described in the Frattini test. The percentage of CaO removed is reported as a representation of the reactivity of material.

Electrical conductivity methods

Electrical conductivity method is an effective technique for evaluation of pozzolanic performance of SCM. It employs the rate of change of conductivity as a parameter to quantify pozzolanicity. A number of previous research studies adopted the conductivity measurement to assess reactivity of pozzolanic materials. A loss in electrical conductivity is observed due to the reaction between calcium hydroxide (in aqueous solution) and the pozzolanic material. Paya et al. (2001) measured the electrical conductivity of pozzolanic materials/lime aqueous suspensions [15]. The loss of conductivity of the suspension varied with time and this variation was directly associated with reactivity of SCM. McCarter and Tran (1996) suggested an efficient activation method for the measurement of pozzolanic activity [16].

According to this method, the pozzolanic material is directly activated with calcium hydroxide. The pozzolanic material is mixed with powdered Ca(OH)2 in the ratio of 8:2. Water/solid ratio is suggested in the range of 0.45-1.2 based on the specific gravity of the pozzolanic materials. The electrical conductance of the mixture is measured at a controlled

temperature for a constant frequency of applied alternating current. Pozzolanic activity is calculated as the ratio of the maximum rate of change of conductivity to the time at which this maximum is reached. The effect of activation in the pozzolanic activity test can be examined using saturated Ca(OH)2 solution or powdered Ca(OH)2 activation. In line with experimental observations, powdered Ca(OH)2 activation was suggested as a more reliable and accurate method for conductivity measurement [10]. Figure 4 shows typical curves for conductivity and rates of conductivity for some mineral admixtures.

Luxan et al. (1989) proposed a quick method to find the reactivity of natural pozzolans by conductivity measurement [17]. In this method, the difference in conductivity right after mixing and after 120 seconds with a pozzolanic material in a saturated lime solution was measured. Although the study showed good results, the



suggested duration for the test is less and this may lead to an inappropriate evaluation. On the other hand, Sinthaworn and Nimityongskul (2009) suggested a quick and reliable method to determine of pozzolanic activity within 28 hours [18]. In this method, a filtrate of OPC solution was prepared (350 g OPC with 1000 ml distilled water). A beaker with 200 ml of OPC solution was placed with magnetic stirrer and subsequently heated to 80oC to accelerate the reaction. One gram of pozzolanic material was added to the solution and electrical conductivity was measured for 28 hours and pozzolanic performance of SCMs was represented in terms of the rate of change of conductivity.

Modified chapelle activity test

This method has been widely used by many researchers. In this test, 1 g of CaO is dissolved in distilled water and diluted to 250 ml. Thus, the initial CaO concentration of the solution is known. 1 g of pozzolanic material is added in the solution and thoroughly mixed for more than one minute. The solution needs to be kept in an oven at 90oC for 16 hours. Afterwards, the filtrate is prepared and titrated with hydrochloric acid using phenolphthalein indicator. From the titration, CaO concentration of the filtrate is determined. Chapelle activity is expressed as the difference between initial CaO concentration and final CaO concentration of the filtrate (determined from the titration).

Thermogravimetric Analysis

Paya et al. (2002) determined reactivity of pozzolanic materials using thermogravimetric monitoring in lime paste and OPC paste. Pozzolanic activity was estimated in terms of reacted lime in the control and pozzolanic materials replaced mixture [19]. Different levels of replacements were suggested based on materials for OPC/Pozzolan mixtures whereas 3 (pozzolan): 7 (hydrated lime) ratio mixes were recommended for lime/pozzolan mixture. Pastes were sealed in airtight bottles and kept at 20ºC for 56 days. After that, the hardened samples were ground manually and acetone was used to terminate the hydration process. Thermogravimetric analysis (TGA) was used to determine pozzolanic reactivity. Generally, thermogravimetric curve and its first derivative curve are obtained. A temperature of 450-550oC is attributed to dehydroxylation of calcium hydroxide due to the pozzolanic performance of SCM. The amount of calcium hydroxide available in the paste is measured as Ca(OH)2 produced during the hydration of cement minus the consumption due to pozzolanic reaction. The fixed lime was calculated for all mixes to evaluate pozzolanic activity of pozzolanic materials. Moropoulou et al. (2004) assessed pozzolanic activity using differential thermal and thermo gravimetric analysis. The reacted Ca(OH)2 was quantified by DTA/TG analysis, and

was suggested as an indicative parameter for pozzolanic activity assessment [20]. Shvarzman et al. (2003) suggested use of TGA with X-ray powder diffraction to estimate degree of dehydroxylation and amorphisation due to pozzolanic activity [21].

WAYS TO ACHIEVE AN EFFICIENT EVALUATION METHOD IN INDIAN STANDARDS

Need to revise current pozzolanic test procedure based on material characteristics

As stated earlier, the Indian standard suggests lime reactivity test method with common procedure (same level of replacement, same curing method and curing duration) for all kinds of supplementary cementitious materials. However, cementitious and pozzolanic materials like slag can be used at higher level of replacement whereas highly pozzolanic materials like silica fume are incorporated with lower range of replacement to obtain desirable fresh and hardened properties of concrete. Additionally, the curing duration need not be same for highly reactive and low reactive pozzolanic materials. The strength activity test method is suggested in ASTM standard with different level of replacements and curing methods based on reactivity and characteristics of materials. In a similar manner, the changes in the curing method and duration need to be addressed in the current procedure of lime reactivity test. Instead of a single test method, the reactivity of SCM can be evaluated by more than one reliable method such as chemical analysis, conductivity measurement, or thermogravimetric analysis to obtain a proper assessment of pozzolanic performance of the SCM.

Need to include characterisation techniques in the assessment procedure

IS 1727 and ASTM C1709 [22] suggest wet chemical analysis for oxide composition. Although conventional wet chemical analysis is appropriate, individual titration procedures are suggested for different oxides, which needs controlled testing procedure, more time consumption, skilled technical supervision and high cost. X-ray fluorescence spectroscopy (XRF) is a reliable and rapid method that uses characteristic X-ray (fluorescent X-ray) to find the elemental composition of the supplementary cementitious materials and from the elemental composition, oxide composition of SCM can be determined. In the same way it can be opted for ordinary Portland cement and blended cements. This method is a more accurate, less expensive and rapid method compared to wet chemical analysis because of possibilities of manual error in the chemical analysis. Use of XRF needs to be addressed in


POINT OF VIEW

Indian standards to attain effective assessment of material characteristics.

Pozzolanic performance of SCM is directly related to its amorphous silica content. Materials that have higher percentage of crystalline silica content (in the form of quartz or crystobalite) cannot be used as pozzolanic materials. It is very important to quantify the phase composition in order to specify reactivity of supplementary cementitious materials. X-ray diffraction technique is an effective tool to study crystallography of materials and phase (mineralogical) composition of pozzolanic materials. In a number of previous research studies, XRD was used to find the degree of pozzolanic activity of SCM in terms of amorphous silica content and calcium hydroxide consumption. However, a suitable method is not suggested in the Indian standards or in ASTM to find reactive silica content which is a primary characteristic of pozzolanic materials. X-ray diffraction technique needs to be included in the evaluation procedure.

Laser diffraction method is a commonly used technique to determine the particle size distribution of fine powders due to its application over a wide range of sizes from submicron to millimeter. In addition, it is a rapid method with good reliability and easy repeatability. The study of particle size

distribution can lead to a more meaningful interpretation of the type of particles in pozzolanic materials compared to the highly empirical Blaine value. ASTM 1709 suggests laser diffraction analysis for particle size distribution in its evaluation procedure and this should also be suggested in the Indian standard.

Need for microstructural characterisation of new SCMs

Another notable drawback in Indian standards and ASTM standards is the absence of microstructural investigation of pozzolanic materials. The performance of pozzolanic materials, especially fresh properties and reactivity, highly depends on the microstructure. For instance, the spherical shape of fly ash particles is extensively reported as the reason for good workability of fly ash blended concrete. To get a clear insight of different particles present in new SCMs and their morphology, Scanning Electron Microscopy (SEM) can be used. In addition, the changes in the microstructure and elemental composition of particles upon various processing methods such as grinding and burning of particles can be observed using SEM. To achieve thorough and scientific understanding of pozzolanic materials, SEM needs to be included in the assessment procedure. The role



of understanding microstructural characteristics to reach clear scientific understanding of the pozzolanic performance of a new supplementary cementitious material is explained in the following passages.

Assessment of pozzolanic characteristics of sugarcane bagasse ash using microstructural characterisation was investigated by the two of the authors of this paper in a separate study. The structure of the different particles present in the raw bagasse ash was further investigated by scanning electron microscopy (SEM) in the secondary electron mode. The micrographs of different particles as well as elemental composition of the detected phases (using energy dispersive X-ray analysis, or EDX) were studied. The sample of raw bagasse ash consists of three different types of particles, specifically, fine burnt particles, coarse fibrous unburnt (CFU) particles and fine fibrous unburnt (FFU) particles as shown in Figure 5.

Most of the observed particles in the micrograph of raw bagasse ash were irregular particles (See Figure 6). A large amount of Si (50%) was found to be present in the observed phases of all irregular particles by EDX analysis. Because of controlled burning at a high temperature of 500-550oC (lesser temperature than maximum fuel capacity of bagasse) in the cogeneration boiler, fibrous particles were burnt partially and disintegrated parts of fibrous particles were clearly observed in the micrograph of raw bagasse ash. Consequently, a number of partially burnt fibrous particles were observed along with silica rich irregular particles in the micrograph. Carbon was observed as a major element (more than 75%) in the elemental composition of fibrous particles by EDX analysis. Fibrous particle was clearly identified in the micrograph of raw bagasse ash because of its larger size when compared to irregular particles. In the same way, several coarse fibrous particles were also identified in the micrograph of raw bagasse ash. It was interesting to note that these fibrous particles had two completely dissimilar types of microstructure. To elucidate microstructure of fibrous particles, these particles were separated from raw bagasse ash by sieving and investigated by scanning electron microscopy. Coarse fibrous particles were mainly seen to have a cellular structure (83% of carbon in the elemental composition) whereas fine fibrous particles had ordered intercellular structure (78% of carbon in the elemental composition). Observed microstructure of FFU was totally different from CFU and fine burnt particles. In earlier research studies, all the fibrous particles were commonly termed in the category of unburnt particles. However, interpretations from the microstructural analysis directly point out the different nature of coarse (CFU) and fine (FFU) fibrous particles. To

procure further scientific insight, the elemental compositions of the intercellular structure and intercellular channels were compared with loss on ignition results (as per IS 1727:2004) of CFU and FFU particles to confirm the presence of higher amount of unburnt carbon and very less silica content (lesser than 2%), which was observed from EDX analysis of different parts of fibrous particles.

Preliminary evaluation of raw bagasse ash as per IS 1727 and ASTM C311 confirmed it as a non pozzolanic material. Loss on ignition of raw bagasse ash was found to be very high (21%) and above the permissible range described in the standard. Pozzolanic activity index of raw bagasse ash was 69% and well below the minimum requirement to define as a supplementary cementitious material. From the microstructural analysis and EDS observations, it can be understood that fine fibrous particles were carbon rich unburnt particles without reactive silica. As a result, the sample of raw bagasse ash had lesser strength than minimum requirement in the strength activity test to define as supplementary cementitious material. The loss on ignition of bagasse ash screened through 300 µm sieve (for complete removal of fibrous particles) was found to be 3%. It is clear that removal of fibrous unburnt particles effectively reduced the loss on ignition, water requirement for a given consistency, and considerably improved its pozzolanic activity to 78% which is well above the minimum requirement. This study


POINT OF VIEW

highlighted the importance of microstructural analysis for the assessment of new supplementary cementitious materials to understand the nature of the materials.

In addition to this, microstructural analysis aids to ascertain the effect of different bagasse ash replacements of cement on workability and the compatibility with superplasticisers in cement paste. The saturation dosage of superplasticiser was found to be increased with level of replacement of bagasse ash (Bahurudeen et al., 2014)[24]. The irregular structure of burnt silica particles, which were present in the processed bagasse ash, was a reason for the considerable reduction in relative fluidity of SCBA based cement paste compared to control paste.

Need for proper guidelines to assess pozzolanic performance of SCM in concrete

Different guidelines are available for the evaluation of new alternative supplementary cementitious materials. IS 1727 (Indian Standard - Methods of test for Pozzolanic materials) describes different tests. ASTM C1709-11 (Standard Guide for evaluation of Alternative Supplementary Cementitious Materials (ASCM) for use in concrete) presents a detailed plan for technical evaluation of new alternative cementitious materials. A comprehensive investigation on the influence of new supplementary cementitious materials on fresh and hardened properties of concrete is suggested to evaluate the suitability of new materials for use in concrete. A number of tests are also included in this stage (concrete mixtures covering different replacement levels of new pozzolanic materials with binder contents ranging from 200 to 400 kg/m3) to assess the performance in concrete. Fresh concrete testing includes slump, air content, and temperature, fresh density, bleeding and setting time. Compressive and flexural strength, modulus of elasticity, drying shrinkage, alkali-silica reaction, permeability, heat of hydration and sulphate resistance are listed in the recommended hardened testing. Apart from material characteristics and lime reactivity test, IS 1727 suggests compressive strength, drying shrinkage, permeability, transverse strength, reduction in alkalinity and silica release for performance evaluation. All these test methods are recommended only for mortar and not for concrete.

Even the permeability test is suggested in pozzolana-cement mortar instead of concrete. Pore refinement and improvement of interfacial transition zone (ITZ) caused by a reduction in the permeability of SCM replaced concrete are widely reported in the existing literature. Thus, permeability studies need to be conducted in concrete instead of mortar specimens. Instead of using a single test, there is a need to

look at the durability performance of SCM based concrete by different test methods, because ingress of aggressive chemicals is based on different transport mechanisms including permeation, adsorption, diffusion, migration, convection, absorption, sorption and combinations of these mechanisms (Alexander et al., 1999) [25]. A systematic and thorough investigation of durability performance needs to be included in the performance evaluation for any new pozzolanic material to enable its potential use in concrete.

Reduction in heat of hydration is another effect that has been reported for most pozzolanic materials by several researchers [26,27]. Heat evolution studies must be included in any test program for the performance of SCMs.

Improper selection or incompatibility of superplasticiser with cement and pozzolanic materials leads to adverse effects on the fresh and hardened properties [28]. Segregation, delayed setting, loss of workability, less workability retention are the common problems observed in incompatible superplasticised concrete [29]. This primarily happens because the incorporation of supplementary cementitious materials in modern high strength concrete leads to greater consumption of superplasticiser when compared to conventional concrete [30,31]. Guidelines need to be included to address proper compatibility criteria of superplasticiser and binder systems with SCMs (sometimes in a ternary or quaternary combination) to achieve desirable fresh properties.

Need for consideration of processing methods

Most of the supplementary cementitious materials are obtained as by-products and waste materials. Direct assessment of pozzolanic performance of raw materials leads to inappropriate evaluation of the material. Evaluation of new pozzolanic materials needs to be extended to include the influence of different processing methods. The changes in the pozzolanic activity with respect to different processing methods need to be comprehensively assessed. Selection of new alternative cementitious materials should be done based on the maximum possible pozzolanic activity of materials with minimum level of processing. Guidelines for the evaluation of new pozzolanic materials should consider suitable systematic procedure for processing.

Various tests for evaluating the performance of a new supplementary cementing material are listed in Table 4 and a proposed scheme for assessment of any pozzolanic material is presented in Figure 7.




POINT OF VIEW

Table 4. List of tests for evaluating the performance of a supplementary cementing material

Performance evaluation of supplementary cementing materials (SCM)

Sl. No.

Description of Test Remarks

I Physical Characterization

1 Moisture content Raw sample2 Specific Gravity 3 Particle Size Distribution Further grinding

carried out if required4 Surface area (Fineness)

5 Loss on Ignition Especially for agro-waste (RHA, SCBA)

II Chemical Characterization

1 Wet chemical analysis To determine and quantify chemical

composition particularly such as silica, calcium oxide, alumina, ferric oxide,

magnesium oxide, alkali-oxides, etc.

2 X-ray Fluorescence

3 Fourier Transform - Infrared Spectroscopy

4 Energy Dispersive X-ray

III Mineralogical Characterization 1 X-ray Diffraction Compulsory

IV Morphological Characterization

1 Optical Microscopy

2 Scanning Electron Microscopy Compulsory

V Assessment of Pozzolanic Activity

1 Mortar Compressive Strength at various ages Compulsory

2 Lime Saturation testMinimum one test

compulsory3 Electrical Conductivity test4 Modified Chapelle activity test5 X-ray Diffraction 6 Scanning Electron Microscopy 7 Petrographic examination 8 Thermogravimetric analysis Optional9 Sorptivity and Porosity test Optional10 Lime Reactivity test OptionalVI SCM based Concrete Performance (i) Fresh Concrete properties 1 Chemical admixture compatibility Compulsory2 Workability Compulsory3 Setting time Compulsory4 Air content

(ii) Hardened Concrete properties

1 Compressive Strength at various ages Compulsory

2 Flexural Strength

3 Durability tests Compulsory

4 Non-Destructive tests

VII Field trials Optional

CONCLUSIONThis paper explains the current specifications in the Indian and International standards for evaluation of supplementary cementitious materials. The need for consideration of processing methods in the evaluation practice to achieve maximum possible pozzolanic performance of new SCM and application of characterisation tools to attain exact characteristics of SCM are highlighted. Furthermore, the requirements to achieve an efficient evaluation scheme for supplementary cementitious materials in the Indian standards are emphasised and a proper characterisation strategy for new SCMs for use in concrete is described in the paper.

References

Standard specification for coal fly ash and raw or calcined natural pozzolan for use in Concrete, ASTM C618-12a (2012), American Society for Testing and Materials, West Conshohocken, United States.Methods of test for pozzolanic materials, IS 1727:1967, (2004), Bureau of Indian Standards, New Delhi, India.Pulverised Fuel Ash - Specification - Part 1 : For Use as Pozzolana in Cement, Cement Mortar and Concrete, IS 3812:2003, Bureau of Indian Standards, New Delhi, India.Portland pozzolana cement Specification - Fly ash based, IS 1489: Part 1 (2005), Bureau of Indian Standards, New Delhi, India.Specification for standard setting quick lime and pozzolana mixture, IS 10772:1983, Bureau of Indian Standards, New Delhi, India.Standard specification for silica fume used in cementitious mixtures, ASTM C1240-12 (2012), American Society for Testing and Materials, West Conshohocken, United States.Specification for calcined clay pozzolana, IS 1344-1981 (2008) Bureau of Indian Standards, New Delhi, India.Specification for Portland slag cement, IS 455-1989 Bureau of Indian Standards, New Delhi, India.Silica fume specifications, IS 15388-2003 Bureau of Indian Standards, New Delhi, India.Specification for granulated slag for manufacture of Portland slag cement, IS 12089: 1987 (2008) Bureau of Indian Standards, New Delhi, India.Standard test methods for sampling and testing fly Ash or natural pozzolans for use in Portland-cement concrete, ASTM C311-11b (2011), American Society for Testing and Materials, West Conshohocken, United States.Standard specification for slag cement for use in concrete and mortars, ASTM C989/C989M−13 (2013), American Society for Testing and Materials, West Conshohocken, United States.Method of testing cement. Pozzolanicity test for pozzolanic cement, BS EN 196-5 (2005), British Standard. United Kingdom.Donatello, S., M. Tyrer, and C.R. Cheeseman (2010) Comparison of test methods to assess pozzolanic activity. Cement and Concrete Composites, 32, 121–127.Paya, J., M.V. Borrachero, J. Monzo, E.P. Mora, and F. Amahjour (2001) Enhanced conductivity measurement techniques for evaluation of fly ash pozzolanic activity. Cement and Concrete Research, 31, 41-49.

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.


POINT OF VIEW

McCarter, W.J. and D. Tran (1996) Monitoring pozzolanic activity by direct activation with calcium hydroxide. Construction and Building Materials, 10(3), 179-184.Luxan, M.P., F. Madruga, and J. Saavedra (1989) Rapid evaluation of pozzolanic activity of natural products by conductivity measurement. Cement and Concrete Research, 19, 63-68.Sinthaworn, S and Nimityongskul.P (2009) Quick Monitoring of Pozzolanic Reactivity of Waste Ash. Waste Management, 29, 1526-1531.Paya, J., J. Monzo, M.V. Borrachero, L.D. Pinzon, and L.M. Ordonez (2002) Sugarcane bagasse ash (SCBA): studies on its properties for reusing in concrete production. Journal of Chemical technology and Biotechnology, 77, 321-325.Moropoulou, A., E. Bakolas, and Aggelakopoulou (2004) Evaluation of pozzolanic activity of natural and artificial pozzolans by thermal analysis. Thermochimica Acta, 420, 135–140.Shvarzman, A., K. Kovler, G. Grader, G. Shter, (2003) The Effect of dehydroxylation /amorphisation degree on pozzolanic activity of kaolinite. Cement and Concrete Research, 33, 405-416ASTM C1709-11 (2011) Standard guide for evaluation of alternative supplementary cementitious materials (ASCM) for use in concrete. American Society for Testing and Materials, West Conshohocken, United States.Bahurudeen, A. and M. Santhanam (2014) Performance evaluation of sugarcane bagasse ash-based cement for durable concrete. Proceedings of 4th International conference on the Durability of concrete structures. Purdue University, West Lafayette, United States, 275-281.

16.

17.

18.

19.

20.

21.

22.

23.

Bahurudeen, A., A.V. Marckson, A. Kishore, and M. Santhanam (2014) Development of sugarcane bagasse ash based Portland pozzolana cement and evaluation of compatibility with superplasticisers. Construction and Building Materials 68 (2014) 465–475.Alexander, M.G., Y. Ballim, and J.R. Mackechnie (1999) Guide to the use of durability indexes for achieving durability in concrete structures. Achieving durable and economic concrete construction in the South African context. Research Monograph, 2, 5-11.Mostafa, N.Y. and P.W. Brown (2005) Heat of hydration of high reactive pozzolans in blended cements: Isothermal conduction calorimetry. Thermochimica Acta, 435, 162–167.Snelson, D.G., S. Wild, and M.O. Farrel (2008) Heat of hydration of Portland Cement–Metakaolin–Fly ash (PC–MK–PFA) blends. Cement and Concrete Research, 38, 832–840.Jolicoeur, C. and M.A. Simard (1998) Chemical admixture-cement interaction: Phenomenology and physico-chemical concepts. Cement and Concrete Composites, 20, 87-101.Jayashree, C. (2009) Study of cement-superplasticiser interaction and its implications for concrete performance. PhD thesis. Indian Institute of Technology Madras, India.Jayasree, C. and R. Gettu (2008) Experimental study of the flow behaviour of superplasticised cement paste. Materials and Structures, 41, 1581-1593.Agullo, L., B.C. Toralles, R. Gettu, and A. Aguado (1999) Fluidity of cement pastes with mineral admixtures and superplasticiser- A study based on the Marsh cone test. Materials and structures, 32, 479-485.

24.

25.

26.

27.

28.

29.

30.

31.

Dr. A. Bahurudeen holds an ME in Structural Engineering from College of Engineering Guindy (CEG), Chennai; PhD from Indian Institute of Technology (IIT) Madras. He is an Assistant Professor at Birla Institute of Technology and Science Pilani (BITS Pilani). His research interest includes supplementary cementing materials, characterisation of concrete and durability.

Hemalatha M.S. holds an ME degree in Geotechnical Engineering from Indian Institute of Science, Bangalore; pursuing her PhD under the guidance of Prof. Manu Santhanam in Building Technology and Construction Management Division, Department of Civil Engineering at IIT Madras, Chennai. She is an Assistant Executive Engineer in Tamil Nadu Public Works Department, Chennai. Her research interests include quantitative evaluation of the performance of supplementary cementing materials in concrete using characterization techniques.

Dr. Manu Santhanam holds a PhD in Civil Engineering from Purdue University, USA, and is a Professor at IIT Madras, Chennai. He has a few years industrial experience in a construction chemicals company and his research interests include special concretes, cement chemistry, durability and non-destructive evaluation.

Documents

Need of an efficient procedure for the evaluation of ... · PDF fileNeed of an efficient procedure for the evaluation of pozzolanic performance of supplementary cementitious materials